Dynamically allocated thread-local storage

ABSTRACT

Dynamically allocated thread storage in a computing device is disclosed. The dynamically allocated thread storage is configured to work with a process including two or more threads. Each thread includes a statically allocated thread-local slot configured to store a table. Each table is configured to include a table slot corresponding with a dynamically allocated thread-local value. A dynamically allocated thread-local instance corresponds with the table slot.

BACKGROUND

Computer applications having concurrent threads executed on multiple processing systems (such as multiple processors, multiple processor cores, or other forms or parallelism) present great promise for increased performance but also present great challenges to developers. The growth of raw sequential processing power has flattened as processor manufacturers have reached roadblocks in providing significant increases to processor clock frequency. Processors continue to evolve, but the current focus for improving processing power is to provide multiple processor cores on a single die to increase processor throughput. Sequential applications, which have previously benefited from increased clock speed, obtain significantly less scaling as the number of processing systems increase. In order to take advantage of multiple processing systems, concurrent (or parallel) applications are written to include concurrent threads distributed over the processing systems.

A process includes one or more threads and the code, data, and other resources of a program in memory. Typical program resources are open files, semaphores, and dynamically allocated memory. A thread is basically a path of execution through a program. A thread typically includes a stack, the state of the processor registers, and an entry in the execution list of the system scheduler. Each thread shares resources of the process. A program executes when the system scheduler gives one of its threads execution control. The scheduler determines which threads will run and when they will run. Threads of lower priority might have to wait while higher priority threads complete their tasks. On multiprocessor machines, the scheduler can move individual threads to different processors to balance the workload. Each thread in a process operates independently. Unless the threads are made visible to each other, the threads execute individually and are unaware of the other threads in a process. Threads sharing common resources, however, coordinate their work by using semaphores or another method of inter-process communication.

Thread Local Storage (TLS) is a method by which each thread in a given multithreaded process can allocate locations in which to store thread-specific data and uses static or global memory local to a thread. Typically all threads in a process share the same address space, which is sometimes undesirable. Data in a static or global variable is typically located at the same memory location, when referred to by threads from the same process. Variables on the stack are local to threads, because each thread has its own stack, residing in a different memory location. Sometimes it is desirable that two threads referring to the same static or global variable are actually referring to different memory locations, thereby making the variable thread-local. If a memory address sized variable can be made thread-local, arbitrarily sized memory blocks can be made thread-local by allocating such a memory block and storing the memory address of that block in a thread-local variable.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

Thread-local storage is a programming construct that is useful in multi-threaded programming A thread-local variable has one memory location for each thread, and each thread will be able to see its own value when accessing the variable. One example scenario where thread-local variables are useful is a two-phase program where each thread first computes a partial result, and then the partial results are combined into a final answer. The thread-local storage can be exposed in a platform with a ThreadStatic attribute. A limitation of ThreadStatic attribute is that each thread-local variable is defined in the program source code and not allocated and released dynamically depending on the input.

The present disclosure is directed to dynamically allocated thread storage in the memory of a computing device. Dynamically allocated thread storage is configured to work with a process including two or more threads. Each thread includes a statically allocated thread-local slot configured to store a table. Each table is configured to include a table slot corresponding with a dynamically allocated thread-local value. A dynamically allocated thread-local instance corresponds with the table slot. Dynamically allocated thread-local slots are implemented on top of statically allocated thread-local slots.

This implementation has several advantages over thread-local variables. For example, the implementation provides for faster reads and writes of thread-local variables. Additionally, the implementation can conserve computer memory. Further, the implementation enables enumeration of the values of all threads for a particular thread-local value.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.

FIG. 1 is a block diagram illustrating an example of a computing device for running, hosting, or developing a hash table that can be accessed by two or more concurrent threads.

FIG. 2 is a schematic diagram illustrating an example implementation of dynamically allocated thread-local slots on top of statically allocated thread-local slots configured in the computing device of FIG. 1.

FIG. 3 is a modified schematic diagram illustrating the example implementation of FIG. 2 during cleanup after a thread-local instance is disposed.

FIGS. 4A, 4B, and 4C are modified schematic diagrams illustrating the example implementation of FIG. 3 in successive stages of a cleanup after a thread exits.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. It is to be understood that features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1 illustrates an exemplary computer system that can be employed in an operating environment such as a distributed computing system or other form of computer network and used to host or run a distributed application included on one or more computer readable storage mediums storing computer executable instructions for controlling a computing device or distributed computing system to perform a method. The computer system can also be used to develop the distributed application and/or provide a serialized description or visualized rendering of the application.

The exemplary computer system includes a computing device, such as computing device 100. In a basic configuration, computing device 100 typically includes a processor system having one or more processing units, i.e., processors 102, and memory 104. Depending on the configuration and type of computing device, memory 104 may be volatile (such as random access memory (RAM)), non-volatile (such as read only memory (ROM), flash memory, etc.), or some combination of the two. This basic configuration is illustrated in FIG. 1 by dashed line 106. The computing device can take one or more of several forms. Such forms include a person computer, a server, a handheld device, a consumer electronic device (such as a video game console), or other.

Computing device 100 can also have additional features or functionality. For example, computing device 100 may also include additional storage (removable and/or non-removable) including, but not limited to, magnetic or optical disks or solid-state memory, or flash storage devices such as removable storage 108 and non-removable storage 110. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any suitable method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Memory 104, removable storage 108 and non-removable storage 110 are all examples of computer storage media. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile discs (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, universal serial bus (USB) flash drive, flash memory card, or other flash storage devices, or any other storage medium that can be used to store the desired information and that can be accessed by computing device 100. Any such computer storage media may be part of computing device 100.

Computing device 100 includes one or more communication connections 114 that allow computing device 100 to communicate with other computers/applications 115. An example communication connection can be an Ethernet interface. In some examples, the computing device can also have one or more additional processors or specialized processors (not shown) to perform processing functions offloaded from the processor 102. Computing device 100 may also include input device(s) 112, such as keyboard, pointing device (e.g., mouse), pen, voice input device, touch input device, etc. Computing device 100 may also include output device(s) 111, such as a display, speakers, printer, or the like.

The computing device 100 can be configured to run an operating system software program and one or more software applications, which make up a system platform. In one example, the computing device 100 includes a software component referred to as a managed, or runtime, environment. The managed environment can be included as part of the operating system or can be included later as a software download. Typically, the managed environment includes pre-coded solutions to common programming problems to aid software developers to create applications, such as software programs, to run in the managed environment. An example of a managed environment can include an application framework or platform available under the trade designation .NET Framework available from Microsoft, Inc. of Redmond, Wash. U.S.A.

The computing device 100 can be coupled to a computer network, which can be classified according to a wide variety of characteristics such as topology, connection method, and scale. A network is a collection of computing devices and possibly other devices interconnected by communications channels that facilitate communications and allows sharing of resources and information among interconnected devices. Examples of computer networks include a local area network, a wide area network, the Internet, or other network.

A computer application configured to execute on the computing device 100 includes at least one process (or task), which is an executing program. Each process provides the resources to execute the program. One or more threads run in the context of the process. A thread is the basic unit to which an operating system allocates time in the processor 102. The thread is the entity within a process that can be scheduled for execution. Threads of a process can share its virtual address space and system resources. Each thread can include exception handlers, a scheduling priority, thread-local storage, a thread identifier, and a thread context (or thread state) until the thread is scheduled. A thread context includes the thread's set of machine registers, the kernel stack, a thread environmental block, and a user stack in the in the address space of the process corresponding with the thread.

In parallel applications, threads can be concurrently executed on the processor 102. Concurrent programming for shared-memory multiprocessors can include the ability for multiple threads to access the same data. The shared-memory model is the most commonly deployed method of multithread communication. Multiple threads execute on multiple processors, multiple processor cores, or other classes of parallelism that are attached to a memory shared between the processors.

Thread-local storage is a programming construct that comes useful in multi-threaded programming An ordinary variable represents a single location in the computer memory, and so all computation threads that access the variable will see and mutate the same value. A thread-local variable, in contrast, has one memory location for each thread, and each thread will see its own value when accessing the variable. One example scenario where thread-local variables are useful is a two-phase program where each thread first computes a partial result, and then the partial results are combined into a final answer. Such problems can often be conveniently expressed using thread-local storage.

The thread-local storage is exposed in a platform with a ThreadStatic attribute. An example of a ThreadStatic attribute in a platform such as .NET Framework is known as ThreadStaticAttribute. A ThreadStaticAttiribute constructor initializes a ThreadStaticAttribute class in C# (C-sharp) syntax such as: public ThreadStaticAttribute. The ThreadStaticAttribute class indicates that the value of a static field is unique for the thread. A static field marked with ThreadStaticAttribute is not shared between threads. Each executing thread has a separate instance of the field, and independently sets and gets values for that field. If the field is accessed on a different thread, it will contain a different value. The ThreadStatic attribute allows the user to annotate a static field as thread-local storage. By using a ThreadStatic attribute, the user can annotate multiple global variables as thread-local. Then, each of those global variables will have one storage slot in each thread in the process, instead of simply having one storage location in total for example.

A limitation of ThreadStatic attribute is that each thread-local variable is defined in the program source code. As a result, thread-local variables are not allocated and released dynamically. Instead, the number of thread-local variables is constant and defined in the program source code.

FIG. 2 illustrates an example implementation 200 of dynamically allocated thread-local slots on top of statically allocated thread-local slots in a computer memory 104 that can be operated on by the processor 102. The example implementation 200 includes two or more threads 202, such as three threads 204, 206, 208 shown each having a statically allocated thread-local slot 210, 212, 214, respectively. The instance of the statically allocated thread-local slot 210, 212, 214, in each thread 204, 206, 208 is configured to store a table 216, 218, 220, respectively. Each thread 204, 206, 208 has a different copy of the thread-local slot 210, 212, 214 so each thread will include a separate table. Each table 216, 218, 220 includes at least one slot 222, 224, 226. In the illustrations, each table 216, 218, 220 includes five slots, i.e., slots 0-4. Each slot 222, 224, 226 in the table 216, 218, 220 can correspond to a dynamically allocated thread-local value.

The example implementation 200 includes at least one dynamically-allocated ThreadLocal instance, such as ThreadLocal instances 228 and 230. ThreadLocal instance 228 has an identifier (ID) of 0, which indicates that values of ThreadLocal instance 228 are stored in slot, or location, 0 of tables 216, 218, 220. ThreadLocal instance 230 has an ID of 3, which indicates that its values are stored in location 3 of the tables 216, 218, 220. Dynamically allocated thread-local values can be implemented on top of statically allocated thread-local values. A platform such as the .NET Framework exposes GetData and SetData methods for allocating thread-local slots at runtime. The “ThreadLocal class” accomplishes the same and provides a convenient interface.

FIG. 3 is a modified schematic diagram illustrating the example implementation 200 demonstrating an example of ThreadLocal instance 230 being removed. (By modified, FIG. 3 does not include an indication of ThreadLocal instance 228 although it can remain in the implementation 200 or even be the subject of the example.) When ThreadLocal instance 230 is removed, corresponding slots, i.e., location 3, are removed. On a platform with garbage collection, the value could be a reference to a large object. Until the reference is cleared, i.e., set to a null value, the memory occupied by the large object will not be released even if no other reference to the large object exists.

A cleanup routine is defined for ThreadLocal when ThreadLocal instance, such as ThreadLocal instance 230 is disposed. The cleanup routine can clear out the table slots associated with the disposed ThreadLocal instance 230. In order to run cleanup routine, however, a linked list 232 is used locate the tables that hold a slot associated with the cleaned up ThreadLocal instance 230, such as tables 216 and 220. Linked list 232 is added into the ThreadLocal instance 230 and enumerates over arrays that hold a value for ThreadLocal instance 230. The linked list 232 thus enables the cleanup routine to locate the tables that hold values for ThreadLocal instance 230.

While the linked list 232 solves the issue of locating the tables that hold values for the ThreadLocal instance 230, the linked list 232 by itself also introduces another issue in that the tables 216, 218, 220 themselves cannot be garbage collected when threads 204, 206, 208 are removed. If the only incoming reference to the tables 216, 220 is a statically allocated thread-local variable, the thread slots 210, 212, 214 can be automatically garbage collected after its owning thread is finished. Now that the thread slots 210, 212, 214 are also a part of the linked list 232, they cannot be garbage collected because the thread slots 210, 212, 214 have an additional incoming reference.

To address this issue, the implementation 200 provides for the back-references to a table to be removed when the corresponding thread 202 is removed with a helper cleanup routine 234 that executes when a thread, such as thread 202, is removed. The helper cleanup routine 234 can be added with a ThreadStatic field in each of the tables 216, 218, 220 that holds an object whose cleanup routine performs the desired cleanup that operates to clear all back references to the tables 216, 218, 220 when the corresponding thread 204, 206, 208 exits.

Thread-local storage can be used to enumerate all values associated with a particular ThreadLocal instance 230, i.e., the values from all threads. For example, values 5 and 7 are associated with the ThreadLocal instance 230. The example implementations in this disclosure can extend to support value enumeration.

To save the values for threads that have exited, the helper cleanup routine 233 can be modified after a thread exits. Before releasing the tables 216, 218, 220, the helper cleanup routine 234 saves the final values into nodes 236, 238 of the linked list 232. The saved values in the linked list 232 can continue to be included in the enumeration of values for the ThreadLocal instance. FIG. 4 shows the steps in the cleanup after a thread has exited.

FIGS. 4A to 4C illustrate an example method of cleanup after a thread has exited from an initial state of the threads 202, 204 and 206 such as the state illustrated n FIG. 3. FIG. 4A illustrates how the linked list 232, second cleanup object 234 and table 216 remain after thread 204 has exited from the initial state illustrated FIG. 3. FIG. 4B illustrates how node 236 of the linked list 232 saves the final value of ThreadLocal instance 230. The second cleanup routine clears the back references of the table 216. Afterwards, in FIG. 4C, the table 216 is garbage collected and the value in node 236 can be enumerated.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof. 

1. A dynamically allocated thread storage in a computing device having a memory, comprising: a plurality of threads included in the memory, wherein each thread includes a statically allocated thread-local slot configured to store a table configured to include a table slot corresponding with a dynamically allocated thread-local value; and a dynamically allocated thread-local instance configured in the memory, the dynamically allocated thread-local instance corresponding with the table slot.
 2. The dynamically allocated thread storage of claim 1 wherein each thread includes a plurality of statically allocated thread-local slots.
 3. The dynamically allocated thread-local storage of claim 1 wherein the dynamically allocated local thread instance enumerates corresponding values.
 4. The dynamically allocated thread-local storage of claim 1 wherein each table includes a plurality of table slots corresponding to a location in each thread, and wherein the dynamically allocated thread instance services each location.
 5. The dynamically allocated thread-local storage of claim 4 and further comprising a plurality of dynamically allocated thread instances each corresponding with a separate location in each thread.
 6. The dynamically allocated thread-local storage of claim 4 wherein the dynamically allocated thread instance includes a linked list corresponding with each location corresponding with a dynamically allocated thread-local value.
 7. The dynamically allocated thread-local storage of claim 6 wherein the linked list is configured to indicate values during a cleanup routine.
 8. The dynamically allocated thread-local storage of claim 7 wherein the linked list includes nodes corresponding with each dynamically allocated thread-local value.
 9. The dynamically allocated thread-local storage of claim 8 wherein the nodes are configured to store the dynamically allocated thread-local value when the thread exits.
 10. The dynamically allocated thread-local storage of claim 7 wherein each thread includes a helper cleanup routine configured to clean up back references to the table.
 11. The dynamically allocated thread-local storage of claim 7 wherein each thread includes another statically allocated thread-local slot configured to store the helper cleanup routine.
 12. A method of dynamically allocating thread storage in a computing device having a memory, the method comprising: processing a plurality of threads included in the memory, wherein each thread includes a statically allocated thread-local slot configured to store a table configured to include a table slot corresponding with a dynamically allocated thread-local value; and generating a dynamically allocated thread-local instance configured in the memory, the dynamically allocated thread-local instance corresponding with the table slot.
 13. The method of claim 12 wherein the table includes a plurality of table slots each configured to correspond with a dynamically allocated thread-local value.
 14. The method of claim 13 wherein the dynamically allocated thread-local instance is configured to correspond with a selected table slot in each of the plurality of threads.
 15. The method of claim 14 comprising a plurality of dynamically allocated thread-local instances, wherein each of the plurality of dynamically allocated thread-local instance are configured to correspond with a separate selected table slot in each of the plurality of threads.
 16. The method of claim 15 wherein the dynamically allocated thread-local instance of the plurality of dynamically allocated thread-local instances includes a linked list having a node corresponding with each value in the selected slots.
 17. The method of claim 15 comprising removing one of the plurality of thread-local instances including removing values in the corresponding selected table slot.
 18. A computer readable storage medium storing a platform including computer executable instructions for controlling a computing device, the platform configured to perform a method comprising: processing a plurality of threads wherein each thread includes a statically allocated thread-local slot configured to store a table; wherein the table is configured to include a table slot corresponding with a dynamically allocated thread-local value; generating a dynamically allocated thread-local instance configured in the memory, the dynamically allocated thread-local instance corresponding with the table slot; indicating the table slot with a linked list to remove the dynamically allocated thread-local instance; and removing back references to the table to remove a thread of the plurality of threads.
 19. The computer readable storage medium of claim 18, the method comprising enumerating values of the dynamically allocated local thread instance.
 20. The computer readable storage medium of claim 18 wherein the linked list includes a node corresponding with a dynamically allocated thread-local value and the method comprising storing the dynamically allocated thread-local value in the node before releasing the table to remove the thread of the plurality of threads. 